Colossal thermomagnetic response in chiral $d$-wave superconductor URu$_2$Si$_2$*

The heavy-fermion compound URu$_2$Si$_2$ exhibits unconventional superconductivity at $T_c$ = 1.45 K deep inside the so-called hidden order phase. An intriguing aspect is that this system has been suggested to be a candidate of a chiral $d$-wave superconductor [1], and possible Weyl-type topological superconducting states have been discussed recently. Here we report on the observation of a highly unusual Nernst signal due to the superconducting fluctuations above $T_c$. The Nernst coefficient is anomalously enhanced (by a factor of $\sim 10^6$) as compared with the theoretically expected value of the Gaussian fluctuations. This colossal Nernst effect intimately reflects the highly unusual superconducting state of URu$_2$Si$_2$. The results invoke possible chiral or Berry-phase fluctuations associated with the broken time-reversal symmetry of the superconducting order parameter [2]. [1]Y. Kasahara et al. Phys. Rev. Lett. 99, 116402 (2007). [2]T. Yamashita et al. Nature Phys. 11, 17 (2015).